In order to protect an electrical device, it is common practice to use, between the two lines of an AC mains, a metal oxide varistor, in particular a zinc oxide varistor, connected in series with a gas discharge tube.
Such a device theoretically operates in the following manner: the gas discharge tube withstands practically the entire AC voltage of the mains. Specifically, the stray capacitance of the discharge tube is of a few picofarads, while the stray capacitance of the varistor is of a few nanofarads. When an overvoltage occurs, the latter causes the gas discharge tube to trigger, the latter being able to be extinguished only if the current, termed secondary current, that passes through it subsequently becomes sufficiently small. It is the resistance of the varistor that ensures that the secondary current is limited and enables the gas discharge tube to be extinguished.
However, protection devices combining varistors and discharge tubes are large and bulky.